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The diameter of the mill is the first consideration. If the diameter is 3 feet versus 6 feet, you can see that the farther the balls can drop, the greater the impact or milling force. The second factor is the amount of time you plan on milling a given batch and the throughput that is required. The third consideration is the hardness of the material you plan on milling. The fourth factor is the final required mesh that you plan to mill down to.

Since the balls basically come in one size, and I haven't measured them, they are about 3" diameter. There are sources for small milling balls for milling pigments and small batches in small ball mills i.e. bench-top mills. Balls wear down and smaller (worn) balls can be found on the market or discarded around old mill sites.

The control variables used in milling are time, rotational speed, and number of balls. The diameter of the mill is fixed, the quantity of charge is a variable that is set by the demands of the milling program. The target mesh size is determined by the requirement of "liberation" of free gold or floatable minerals in sulfide - flotation circuits or leach permeability of the rock in a milling - leach circuit.